10 



INTRODUCTION TO ZOOLOGY 



whilst another form, with shorter pseudopodia and several 

 nuclei, is distinguished as Amoeba quarta. 



Unicellular Forms which secrete a Shell. 



Many forms closely allied to Amoeba have taken one step 

 upward on the ladder of life, by acquiring the habit of 



secreting round their 

 bodies a covering, which 

 is not temporary as in 

 the case of the Amoeba's 

 "cyst," but is permanent, 

 and is of such a form, that 

 whilst allowing all vital 

 processes to continue un- 

 checked, it serves as some 

 slight protection to the 

 soft protoplasmic body. 

 These protective "shells" 

 are usually of calcium 

 carbonate (chalk), or of 

 silica, and they are per- 

 forated by one or more 

 pores through which the 

 protoplasm can freely 

 project; incidentally they 

 exhibit a wonderful 

 variety and beauty of 



FIG. 2. Shells from a Chalk Formation. 



form. 



Formation -A- few of these are shown in Fig. 2. The 



of Chalk special forms illustrated are all taken from sea- 



Rock. W ater forms, which, when alive, float freely in the 



water, but at death sink slowly to the bottom of the sea, 



where their chalky shells may accumulate, year after year, for 



many centuries, forming a thick layer, such as is now being 



deposited on the bed of the Atlantic Ocean, where this is not 



at a greater depth than 12,000 feet. 1 



1 At a greater depth than this, only siliceous shells are found, for the 

 delicate calcareous shells become dissolved by the carbonic acid in the water 

 which has a greater solvent power at the increased pressure resulting from 

 the greater depth below the surface. 



